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The Climate Impacts Group (CIG) translates global-scale climate forecasts
and conditions into regional-scale climate forecasts for Pacific
Northwest (PNW) resource managers and the general public. The
El Niño/Southern Oscillation (ENSO) is
the most important factor for seasonal forecasting, changing the odds
for different types of winter and spring weather (e.g. warmer/drier, cooler/wetter)
in the PNW. Another important climate variable for Pacific Northwest climate is the Pacific Decadal Oscillation (PDO). The climate outlook also provides the basis for natural resource
forecasts, including the CIG's annual streamflow forecasts.

What's Next for the Pacific Northwest?

January 2009
Updated January 23, 2009 (posted January 25)

The climate outlook is reviewed monthly and updated as needed.

The Pacific Northwest (Oregon, Washington, and Idaho; PNW)
experienced a wide variety of weather in the last 30 days, from the
cold and snow of late December to the mild temperatures, heavy rains, and
flooding of early January, to an extended strong inversion in mid-January where a layer of warmer air trapped cold,
stagnant air at the
surface. The temperature
records over the region document the geographical extent of the different episodes (Seattle/Tacoma; Lewiston, Idaho; Pocatello (southeast) Idaho; Eugene Oregon (CPC)). The heavy
precipitation of early January was mostly in Washington and
central northern Idaho
(map, 1971-2000 mean, HPRCC). More details on the associated flooding is available through
the Office of the Washington
State Climatologist. Additionally,
Professor Cliff
Mass at the University of Washington provides numerous analyses of the individual weather
events during this period on his weather blog.

Seasonal mean temperatures since the beginning on the Water Year on 1 October have been near or slightly above
normal (1971-2000). Total seasonal precipitation was dominated by the heavy
precipitation of early January in Washington and northern Idaho
(compare with the
mid-December analysis). Snow pack, as measured in
terms of snow water equivalent, is 10-25% above normal on the
western flanks of the Cascades and 25-50% below normal on the
eastern flanks, while a range of
conditions exist over the remainder of the Columbia Basin (22 January analysis,
map legend,
current analysis, NWRFC). Despite the warm
rains of early January, the snow conditions are much improved
over those reported in mid-December.

The ocean surface temperatures during the same period (map, ESRL) are characterized by a
continuation of cooler than normal sea surface temperatures
(SSTs) along the west coast of North America and on the central
and eastern equatorial Pacific, Above normal SSTs are found in the
central north Pacific. The pattern in the north Pacific projects
onto the negative polarity of the Pacific Decadal Oscillation
(PDO). The December SST structure near the Washington, Oregon, and California
coasts at 9km resolution (analysis, PFEL) is characterized by small,
slightly positive anomalies along the Washington and Oregon
coasts, and colder than normal SSTs along the California coast.

El
Niño/Southern Oscillation
(ENSO). The monthly mean Niño 3.4 region (5°N-5°S, 170-120°W)
SST anomalies were weakly negative in September,
October, and November (average -0.23°C, 1971-2000
mean) and the anomalies intensified in December
and early January (December average -0.73°C). In
early January the NOAA Climate Prediction Center
declared that the ocean was in "La Niña conditions" (defined as a monthly
average anomaly less than -0.5°C along with constant atmospheric
conditions and a forecast of persistence for 3
consecutive months; panel 21 of
document).
This declaration is consistent with the
expectation that the ocean will evolve into a "La
Niña episode" by the end of January, which
requires a 3-month mean SST anomaly of less than -0.5°C (more information). The average
February-March-April forecast anomaly of the 23
ENSO forecast models polled by the International Research Institute for
Climate and Society is -0.5°C, just
meeting the "La Niña episode" criterion,
with the anomalies forecast to diminish in
subsequent months.

Pacific Decadal
Oscillation (PDO). The
PDO has exhibited negative monthly values since
September 2007 (digital values), with values in excess of -1
standard deviation from April through November of
2008. [For a normally distributed variable, only
32% of the values exceed one standard deviation in
magnitude.] In December 2008 the strength of the
PDO was slightly diminished with an index value of
-0.87 standard deviations. "La Niña"
is consistent with the development or continuation
of negative PDO
values. The NOAA Earth System Research
Laboratory's statistical Linear Inverse Model PDO
forecast is for continued cold polarity, but of
smaller magnitude, through the end of the forecast
period (December 2009) (forecast). The NOAA coupled
forecast system model solves dynamical equations for
ocean and atmosphere motions, and it predicts
significant cold anomalies along the west coast of
North America through April-May-June, with
diminished anomalies thereafter (forecast).

The lowland snows of December occurred at a time of significant
negative amplitude of the Pacific/North American (PNA) pattern.
Earlier studies have documented an increased chance of heavy snow and extreme cold
during these periods (more information).
The NOAA Climate Prediction Center (CPC) makes 14-day PNA
forecasts
(through 6 February, beyond 6 February). The PNA is presently negative and the red curves in the top panel indicate that the negative PNA polarity will persist through the forecast period.

The longer-term February-March-April CPC climate forecast is for a greater than 33% chance of
below normal temperatures north and west of Pocatello, Idaho,
with the probabilities exceeding 40% in northern Oregon,
northernmost Idaho, and all of Washington (map). The precipitation forecast for the same
months is for an even chance of below, equal to, and above
normal precipitation throughout the PNW (map). The skill of the
precipitation forecast is derived from ENSO-related changes in
precipitation, and the model has no useful skill in ENSO neutral
periods. The forecasts should be interpreted as the tilting of odds
towards general categories of conditions, and should not be viewed
as a guarantee that the specified conditions will be realized.